The present invention relates to the use of supercritical or near supercritical carbon dioxide or propane to remove polychlorinated dibenzo-p-dioxins, (referred to hereinafter as "PCDD's" or simply "dioxins"), and polychlorinated dibenzofurans, (referred to hereinafter as "PCDF's") and to remove sticky contaminants (hereinafter referred to as "stickies") from secondary fibers.
As known in the art, secondary fibers comprise materials, usually cellulose-based, which have been used at least once in their intended primary use area but are, nevertheless, amenable to further processing and subsequent reuse. Waste paper, newsprint, ledger stock, packaging materials, cartons, boxes, computer printouts, telephone directories, corrugated boards, and the like represent suitable raw stock for conversion to secondary fibers. The pattern of reuse (i.e., use of the secondary fiber) may not always be similar to the use to which the primary (virgin) fiber was put.
Efficient management of solid wastes, of which cellulose-based materials constitute a significant part (e.g., waste paper, 40%; yard waste, 18%), has become an important societal theme. In recent years, efforts to recycle waste paper have intensified with the ever increasing concerns as to the rate of use of raw materials and the possible adverse environmental impact of common industrial processes. Novel screening systems and sophisticated flotation techniques have emerged which in large measure have successfully addressed the problem of deinking printed stock. New bleaching sequences which avoid the use of chlorine or chlorine compounds and rely solely upon hydrogen peroxide, dithionites, or formamidine sulfinic acid for attaining acceptable levels of brightness are also making their appearance.
One aspect of waste paper reuse, however, has remained a continuing concern. This area is the presence of small quantities of toxic compounds, in particular, PCDD's and PCDF's, in waste papers.
Bleached kraft fibers under a variety of guises (e.g., coated paper, ledger paper, etc.) are often present in substantial quantities in waste paper stock as purchased from commercial dealers. Kraft pulps, when bleached with sequences including an elemental chlorine stage, can contain small but detectable levels of PCDD's and PCDF's. The processing steps currently used to treat waste paper (e.g., pulping/screening/flotation/bleaching) are not effective in removing such compounds from stock containing chlorine-bleached fibers.
PCDD's and PCDF's are large groups of chloro-organic compounds which have become ubiquitous in industrial societies. The structures of these compounds are as follows, where in each case x+y=1-8: ##STR1##
Of the various possible isomers of these compounds, the following are reportedly the most toxic:
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) PA0 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PCDD) PA0 2,3,7,8-tetrachlorodibenzofuran (TCDF) PA0 1,2,3,7,8-pentachlorodibenzofuran (PCDF) PA0 2,3,4,7,8-pentachlorodibenzofuran (pCDF). PA0 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin (HCDD) PA0 1,2,3,7,8,9-hexachlorodibenzo-p-dioxin (HCDD) PA0 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HCDD) PA0 1,2,3,6,7,8-hexachlorodibenzofuran (HCDF) PA0 1,2,3,7,8,9-hexachlorodibenzofuran (HCDF) PA0 1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) PA0 2,3,4,6,7,8-hexachlorodibenzofuran (HCDF) PA0 2,3,7,8-tetrachlorodibenzo-p-dioxin PA0 1,2,3,7,8-pentachlorodibenzo-p-dioxin PA0 2,3,7,8-tetrachlorodibenzofuran PA0 1,2,3,7,8-pentachlorodibenzofuran PA0 2,3,4,7,8-pentachlorodibenzofuran PA0 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin PA0 1,2,3,7,8,9-hexachlorodibenzo-p-dioxin PA0 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin PA0 1,2,3,6,7,8-hexachlorodibenzofuran PA0 1,2,3,7,8,9-hexachlorodibenzofuran PA0 1,2,3,4,7,8-hexachlorodibenzofuran PA0 2,3,4,6,7,8-hexachlorodibenzofuran.
Also reported to be highly toxic are:
See Rappe et al., "Analysis of Polychlorinated Dibenzofurans and Dioxins in Ecological Samples", in Chlorinated Dioxins and Dibenzofurans in the Total Environment II, Keith, L. H., Rappe, C. and Choudhary, G., eds., Butterworh Publishers, Boston, Mass., 1985, pages 125-126.
In the recent past, the issue of health hazards associated with PCDD's and PCDF's has received much attention in the news media. PCDD's and PCDF's are known to cause a temporary form of a skin ailment known as "chlor-acne." Also, PCDD's and PCDF's (particularly 2,3,7,8-TCDD) have been proven to be extremely toxic to certain animals in laboratory studies, in particular to guinea pigs (LD50=0.6-4.0 micrograms/kilogram). See, for example, Ottoboni, A., The Dose Makes the Poison, Vincente Books, Berkeley, Calif., 1984, and Dioxins, NTIS Report No. PB82-136847, Industrial Environmental Research Laboratory, Cincinnati, Ohio, November, 1980, Section 6.
Because of this reported high level of toxicity to a common laboratory test animal (i.e., the guinea pig), there is a general concern as to the long-term effects of PCDD's and PCDF's on human physiology. Accordingly, there is an important need to remove or substantially reduce the content of PCDD's and PCDF's from waste paper stock as part of the recycling process. It is an object of the present invention to respond to this need.
Another aspect of waste paper reuse has remained a continuing problem both with regard to machine operability and with regard to product quality. This area is the presence of sticky contaminants in the stock used to prepare the secondary fibers.
Stickies consist primarily of organic polymers used in the paper converting industry, such as, hot melts, pressure-sensitive adhesives, styrofoam, and latices. Typical stickies include: polyvinyl acetate (PVA) polymers and copolymers, ethylene vinyl acetate (EVA) polymers and copolymers, polystyrene, styrene-butadiene, polypropylene, polyethylene, polyamide, latex and other rubber compounds, wax, and the like. A particularly common source of stickies is the tackifiers which are added to paper products to improve adhesion properties.
When waste papers containing these adhesives/tackifiers are defibered, stickies are broken down into particles having a wide range of sizes, e.g., less than 0.074 millimeter to greater than 0.42 millimeter. Inefficient removal of stickies causes off-quality paper (e.g., poor appearance, lower strength, and/or holes) and paper machine downtime (e.g., web breaks, slippery sheets, and/or deposition of stickies on such machine components as wires, felts, presses, rolls, and/or drying cylinders).
In an effort to deal with these problems various chemical and mechanical means have been considered. For example, talc and zirconium oxide have been used as pacification agents for stickies. High temperatures, high pressures, and/or solvent addition have been employed as dispersion techniques. Reverse hydroclones and throughflow cleaners have been used to try to screen and/or clean stickies from the feed stock.
Also, efforts have been made to select the stock used for recycling ("furnish selection"). In general, furnish selection is not cost effective. Also, as fiber recycling becomes more important in the context of environmental concerns, the furnish itself will tend to become less "virgin" and more of secondary and tertiary origin, which will, in turn, seriously aggravate the stickies problem.
Notwithstanding these wide ranging efforts, the removal of stickies remains an unsolved problem in the area of fiber recycling. As recognized in the art, irrespective of the technique used, some stickies end up in the paper machine headbox where they adversely affect machine runability and product quality. Accordingly, there is an important and continuing need to remove or substantially reduce the content of stickies in waste paper stock which is going to be recycled. It is an object of the present invention to respond to this need.
Supercritical fluids, including supercritical carbon dioxide and propane, have been proposed for use in various industrial and pollution control processes. A review of this work can be found in Eckert et al., "Supercritical fluid processing", Environ. Sci. Technol., 1986, vol. 20, pp. 319-325. Among other things, these authors describe general applications of supercritical fluid technology to materials processing and pollution control. In particular, they discuss a study in which supercritical ethylene was used to remove trichlorophenol from soil as a model for the removal of dioxins and polychlorinated biphenyls (PCBs). No data are presented for dioxins. More particularly, the reference does not disclose or suggest removing dioxins from recycled paper using supercritical carbon dioxide or propane.
Pang et al., "Supercritical Extraction of Aromatic Hydrocarbon Solids and Tar and Bitumens", Ind. Eng. Chem. Process. Des. Dev., 1985, vol. 24, pp. 1027-1032 discuss the use of various supercritical fluids, including carbon dioxide, to extract organic materials from tar sands. The reference mentions the possibility of using supercritical extraction to remove hazardous materials such as PCBs and dioxin from soils. Again, no data are presented for dioxins and no suggestion is made that supercritical carbon dioxide can be used to remove dioxins from secondary fibers. Also, the data presented for tar sands shows that carbon dioxide extraction produced the poorest yield.
Other studies involving the use of supercritical fluids including carbon dioxide to remove hazardous organic materials from environmental solids such as soil can be found in Groves et al. "State-of-the-art on the supercritical extraction of organics from hazardous wastes", CRC Critical Reviews in Environmental Control, 1985, vol. 15, pp. 237-274; Hawthorne et al., "Extraction and Recovery of Polycyclic Aromatic Hydrocarbons from Environmental Solids Using Supercritical Fluids", Anal. Chem., 1987, vol. 59, pp. 1705-1708; Dooley et al., "Supercritical Fluid Extraction and Catalytic Oxidation of Toxic Organics from Soils", EPA Report No. 600/9-87/018F, pp. 383-397; and Brady et al. "Supercritical Extraction of Toxic Organics from Soils", Ind. Eng. Chem. Res., 1987, vol. 26, pp. 261-268. None of these studies discloses or suggests the use of supercritical carbon dioxide or propane to remove PCDD's and PCDF's from secondary fibers.
Along these same lines, U.S. Pat. Nos. 4,338,199 and 4,543,190 to Modell describe a process in which organic materials are oxidized in supercritical water. The '199 patent includes a general statement that its process can be used to remove toxic chemicals from the wastes generated by a variety of industries including forest product wastes and paper and pulp mill wastes. No specific mention is made of dioxins. The '190 patent describes the treatment of various chlorinated organics other than dioxins with supercritical water and states that conversion of these materials to chlorinated dibenzo-p-dioxins was not observed (see Example 6). The use of supercritical water to treat organic waste materials is also disclosed in PCT Patent Publication No. WO 81/00854, Modell et al., U.S. Pat. No. 4,113,446, Burleson, U.S. Pat. No. 4,564,458, and Titmas, U.S. Pat. No. 4,594,164.
A summary of experiments performed by Modar, Inc., using the Modell supercritical water process was published in Chemosphere--Chlorinated Dioxins and Related Compounds 1987, McNelis et al., editors, Pergamon Press, New York, 1989, Vol. 18, Nos. 1-6, page 50. As described therein, bench-scale tests were performed on soils and liquid wastes contaminated with chlorobenzenes and PCDD's. Supercritical water oxidation was found to remove 2,3,7,8-TCDD and chlorobenzenes from soil and to remove 2,3,7,8-TCDD, TCDD's and OCDD from liquid wastes.
Various uses of supercritical fluids in the processing of materials have been disclosed in the literature. For example, supercritical carbon dioxide has been used to remove tall oil and turpentine from coniferous woods in Fremont, U.S. Pat. No. 4,308,200, to extract lignin from the black liquor produced by the Kraft process for pulp production in Avedesian, U.S. Pat. No. 4,493,797, to treat refinery sludges in European Patent Publication No. 314,223, to regenerate absorbents used in waste water treatment systems in Modell, U.S. Pat. Nos. 4,061,566 and 4,147,624, to sterilize pharmaceuticals in Pilz et al., U.S. Pat. No. 4,263,253, to remove off-flavor materials from textured vegetable products in Sevenants, U.S. Pat. No. 4,675,198, to remove gamma-linolenic acid from fruit seeds in Traitler et al., U.S. Pat. No. 4,703,060, and to decaffeinate coffee in Katz, U.S. Pat. No. 4,472,442; Toro et al., U.S. Pat. No. 4,728,525 and Kaleda et al., U.S. Pat. No. 4,767,634. See also, Friedrich, U.S. Pat. No. 4,466,923; Lawson et al., U.S. Pat. No. 4,495,095; Myerson, U.S. Pat. No. 4,550,198; Panzner et al., U.S. Pat. No. 4,554,170; Japikse et al., U.S. Pat. No. 4,647,466; Ritter and Campbell, "The Effects of Supercritical Carbon Dioxide Extraction on Pine Wood Structure", Biotechnology and Bioengineering Symp., 1986, no. 17, pp. 179-182; Hatakeda et al., "Extraction of Sugi (Cryptomeria japonica D. Don) with supercritical carbon dioxide", Nipon Kagaku Kaishi, 1987, no. 5, pp. 931-933; Shishikura et al., "Concentration of Tocopherols from Soybean Sludge by Supercritical Fluid Extraction", J. Jpn. Oil Chem. Soc., 1988, vol. 37, pp. 8-12; and Li and Kiran "Interaction of Supercritical Fluids with Lignocellulosic Materials", Ind. Eng. Chem. Res., 1988, Vol. 27, pp. 1301-1312.
Supercritical water or near supercritical water has been used to treat wood chips and black liquor from pulping in Modell, PCT Patent Publication No. WO 81/00855. See also Modell, M., "Gasification and Liquefaction of Forest Products in Supercritical Water", Fundam. Thermochem. Biomass Convers., 1985, pp. 95-119; and West et al., "Pyrolysis of 1,3-butanediol as a model reaction for wood liquefaction in supercritical water", Can. J. Chem. Eng., 1987, vol. 65, pp. 645-650.
In addition to their use in waste treatment and materials processing, supercritical fluids have been used in connection with various analytic procedures. For example, Suprex Publication No. TN-022, Suprex Corporation, Pittsburgh, Pa., 1989, mentions the use of supercritical carbon dioxide as part of an analytical procedure for assaying dioxins, but no mention is made of using supercritical carbon dioxide for the reduction of dioxins in cellulosic substrates. Similarly, Hawthorne et al., "Directly Coupled Supercritical Fluid Extraction-Gas Chromatographic Analysis of Polycyclic Aromatic Hydrocarbons and Polychlorinated Biphenyls from Environmental Solids", J. Chromatogr., 1987, vol. 403, pp. 63-76, discuss the use of supercritical fluid extraction coupled to a gas chromatograph to analyze environmental solids, e.g., urban dust, for organic pollutants, specifically, polycyclic aromatic hydrocarbons. The extraction was performed using nitrous oxide as the supercritical fluid. Along similar lines, Schneiderman et al., "Determination of Anthraquinone in Paper and Wood Using Supercritical Fluid Extraction and High-Performance Liquid Chromatography with Electrochemical Detection", J. Chromatogr., 1987, vol. 409, pp. 343-353, describe the combination of supercritical fluid extraction using carbon dioxide, high-performance liquid chromatography, and electrochemical detection to analyze Kraft paper and pine plywood sawdust for anthraquinone.
Significantly, none of these references in any way discloses or suggests that the problems of removing PCDD's and PCDF's or removing stickies from secondary fibers can be solved by treating the fibers with supercritical carbon dioxide or supercritical propane.
Some references exist in the literature regarding attempts to decompose or destroy dioxins in a state of solution in liquid media (e.g., hexane) or in substances such as silica gel or clay via photolytic techniques, e.g., UV radiation. See Ottoboni, supra; Crosby, D. G., et al., Science, Vol. 173, Aug. 20, 1971, pages 173-174; Plimmer, J. R., Bull. Environm. Contam. Toxicol., Vol. 20, 1978, pages 87-92; Botre, Claudio, Adriana Memoil, and Franco Alhaique, Environmental Science and Technol., Vol. 12, No. 3, March 1978, pages 335-336; Crosby, D. G., et. al., Environmental Health Perspectives, Sept. 1973, pages 259-266; Dulin, David, Howard Drossman, and Theodore Mill, Environ. Sci. Technol., Vol. 20, No. 1, 1986, pages 72-77; and Podoll, R. Thomas, Helen M. Jaber, and Theodore Mill, Environ. Sci. Technol., Vol. 20, No. 5, 1986, pages 490-492. The process has been shown to work to an extent but appears to be highly dependent upon the presence of a hydrogen donor solvent, the type and level of impurities present, and the substrate. Furthermore, the photoproduct resulting from irradiation of 2,3,7,8-TCDD has been reported to be trichloro- and dichloro-benzo-p-dioxins, which are less toxic than 2,3,7,8-TCDD but, nevertheless, are undesirable.
As with the art relating to supercritical fluid technology, these references do not address the secondary fiber problem and, in particular, do not suggest that this problem can be solved by extracting PCDD's and PCDF's or by removing stickies from such fibers using supercritical carbon dioxide or supercritical propane.
In considering the particular problem of removing PCDD's and PCDF's from secondary fibers, it is important to note various characteristics of secondary fibers and of PCDD's and PCDF's which make the removal process particularly difficult. For example, secondary fibers have a relatively high surface area per gram. As known in the art, high surface area materials are capable of strongly binding organic compounds, such as PCDD's and PCDF's. See Srinivasan et al., "Binding of OCDD, 2,3,7,8-TCDD and HCB to Clay-Based Sorbents," in Chlorinated Dioxins and Dibenzofurans in Perspective, Rappe, C., Choudhary, G., and Keith, L. H., eds., Lewis Publishers, Inc., Chelsea, Mich., 1986, page 532.
Moreover, in an air or water system, PCDD's and PCDF's will adhere to solid sorbents rather than remaining free in solution. Thus, partition coefficients in the range of 2.8-67.1.times.10.sup.3 have been reported for 2,3,7,8-TCDD for a variety of sorbents including hydroxy aluminum-clay and activated carbon. This compound also adheres quite well to glass in a water environment. See Srinivasan et al., supra at pages 531-537.
The combined effects of large surface areas and large partition coefficients make the effective removal of PCDD's and PCDF's from secondary fibers difficult to achieve. It is to this challenge that one embodiment of the present invention is directed. Alternatively, in a second embodiment, the invention is directed to the removal of stickies from secondary fibers.